Droughts kill trees. That much we know. But beyond the simple concept
of "trees die without water," scientists actually don't know
much about how heat and droughts affect trees. How long can they
go without water? Which ones will die first?

Ecologists at the Los Alamos National Laboratory in New Mexico want to
help trees survive droughts, but first, they have to learn how to kill
them:

Nate McDowell runs what you might call a "tree torture" lab.
It's actually outside in the desert, near the national lab. He's growing
a group of pinon and juniper trees, about 15 feet high. Plastic gutters
keep rain away from the tree roots, to simulate drought. The trees themselves
are growing inside clear plastic chambers — tubes with no tops.
Silvery hoses carry heated air into the chambers.

We climb in through a hole in the chamber where you can immediately
feel the heat. It's about 7 degrees hotter than the outside, roughly
the increase predicted by computer models of climate change over the
next 80 years or so.

McDowell is simulating drought and a warmer climate. He measures how
the trees respond — there are instruments stuck into and all over
the trees. Even wrapped around the stem.

"Every few minutes they measure the diameter of that tree,"
he explains. The trees look like patients in intensive care —
wired up with tubes coming out of the stems. All to see what it takes
to kill it. "Everyone knows it gets hot and dry; you know, beetles
show up, the trees are dead," McDowell says, "but we don't
really understand it."